The background description provided herein is for the purpose of generally presenting the context of the present invention. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely as a result of its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions.
As IP network data services grow rapidly, the demands of operators for transmission capacity increase continuously. The capacity of a single optical fiber in the present network has been gradually approaching the limit value 100 Tbps, and the 100 G transmission system has begun to enter the first year of a commercial era. How to further increase the transmission capacity based on the 100 G transmission signal is the focus of attention of each system equipment producer and operator.
Coherent reception and digital signal processing (DSP) technique are adopted for the receiving terminal in 100 G and 100 G beyond systems, which can digitally compensate the dispersion and polarization mode dispersion (PMD) accumulated in the whole transmission process in the electric field; and polarization modes are reused and various high-order modulation modes are adopted to reduce the Baud rate of the signal, such as PM-QPSK, PDM-16QAM, PDM-32QAM, and even PDM-64QAM and CO-OFDM. However, high-order modulation modes are very sensitive to the nonlinear effect, and therefore a higher requirement is put forward for the optical signal to noise ratio (OSNR). Introducing an optical fiber with low loss and large effective area can improve the OSNR and reduce the nonlinear effect for the system. When a high power density system is adopted, the nonlinear coefficient is a parameter used for evaluating the system performance advantages and disadvantages caused by the nonlinear effect, which is defined as n2/Aeff, where n2 is the nonlinear refractive index of the transmission optical fiber, and Aeff is the effective area of the transmission optical fiber. Increasing the effective area of the transmission optical fiber can reduce the nonlinear effect in the optical fiber.
At present, the effective area of a conventional single-mode optical fiber used for the land transmission system line is only about 80 μm2, while the long-distance land transmission system requires a higher effective area of the optical fiber, and generally it is 100 μm2 or larger. To reduce the costs for laying and the use of repeaters wherever possible, the effective area of the transmission optical fiber is preferably 130 μm2 or larger in the repeaterless transmission system such as a submarine transmission system. However, in the current design for the refractive index profile of the optical fiber with large effective area, a large effective area is often obtained by increasing the diameter of the optical core used for transmitting optical signals. There are some difficulties in the design for such a scheme. On one hand, the basic performance of the optical fiber is primarily determined by the core of the optical fiber and the cladding close to the core, which occupy a large proportion in the manufacturing costs for the optical fiber. If the designed radial dimension is too large, the manufacturing costs of the optical fiber will surely be improved and the price of the optical fiber will also be raised, which will hinder the universal use of such an optical fiber; and on the other hand, compared with a conventional single-mode optical fiber, an increase in the effective area of the optical fiber will result in deterioration of some other parameters, for example, the cutoff wavelength of the optical fiber will increase, but if the cutoff wavelength is too large, then it will be difficult to ensure that the optical signal is under a single mode condition in the transmission waveband of the optical fiber; and in addition, the improper design of the refractive index profile will lead to deterioration of parameters such as bend property and dispersion.
Another optical fiber characteristic that restricts the long distance and high capacity transmission is attenuation. At present, the attenuation of the conventional G.652.D optical fiber is 0.20 dB/km, and the laser energy is reduced gradually after long-distance transmission, so it is necessary to amplify the signal again by using repeaters. In comparison with to the costs for optical fibers and cables, the related equipment and maintenance costs to the repeater station account for 70% or more of the whole link system, so that if a low attenuation or ultra low attenuation optical fiber is involved, then the transmission distance can be extended effectively and the construction and maintenance costs be reduced. According to related calculation, if the attenuation of the optical fiber is reduced to 0.16 dB/km from 0.20 dB/km, the construction cost for the whole link will generally be reduced by about 30%.
To sum up, the development and design of an optical fiber with ultra low attenuation and large effective area becomes an important subject in the manufacturing field of optical fibers. U.S. Publication No. US2010/022533 provides a design of an optical fiber with large effective area, wherein in order to obtain a lower Rayleigh coefficient, the design of a pure silica core is adopted. In such a design, the core is not co-doped with germanium and fluorine, and the fluorine-doped silica is used as the outer cladding. For such a design of a pure silica core, a complicated viscosity matching must be conducted inside the optical fiber, and a very low speed is required in the drawing process, so as to avoid attenuation increase caused by defects inside the optical fiber due to high-speed drawing, so the manufacturing process is extremely complicated.
European Patent No. EP2312350 provides a design of an optical fiber with large effective area and non-pure silica core, wherein a ladder-like sunken cladding structure is adopted for the design, and a pure silicon dioxide outer cladding structure is adopted for another design. The relevant performance can meet the requirements of the optical fiber with large effective area G.654.B and D, but in the design the maximum radius of the fluorine-doped cladding portion is 36 μm, therefore although the cutoff wavelength of the optical fiber can be enabled to be less than or equal to 1530 nm, the micro and macrobend properties of the optical fiber will become poor due to the influence of the small fluorine doping radius, so that in the cabling process of the optical fiber, and the attenuation will increase. In addition, relevant bend properties are not mentioned in the document.
Chinese Patent No. CN10232392 A describes an optical fiber with larger effective area. The effective area of the optical fiber disclosed in the invention reaches 150 μm2 or larger, but the conventional design of the core which is co-doped with germanium and fluorine is adopted, and such a design is achieved at the expense of the performance indexes of the cutoff wavelength; the allowable cutoff wavelength of an optical cable is 1450 nm or more, and in the embodiments of the invention, the cable cutoff wavelength even reaches 1800 nm or more. In practical application, if the cutoff wavelength is too large, then it will be hard to ensure that the optical fiber can be cut off at the applied waveband, and unable to ensure that the optical signal is under a single mode condition during transmission. Therefore, a series of practical problems may be faced when such an optical fiber is used. In addition, in the embodiments of the invention, the minimum outer diameter of the sunken cladding r3 is 16.3 μm, which is also too large. The optimal combination of parameters (such as effective area and cutoff wavelength) of the optical fiber and manufacturing costs of the optical fiber is not obtained in the invention.
For the profile design and manufacturing methods of the conventional optical fibers, the core is co-doped with plenty of Ge/F, and in order to obtain the best macrobend performance, the relative refractive index of the core is often greater than 0.35%, i.e., the core is doped with more Ge, and therefore the Rayleigh scattering will be large to result in attenuation increase of the optical fiber.
Chinese Application No. CN201310394404 provides a design of an optical fiber with ultra low attenuation, wherein the design of a pure silicon dioxide outer cladding is used, but because a typical step profile structure is used, a trench cladding design is not used for optimizing the bend properties of the optical fiber, and the core is not doped with Ge, so that a viscosity mismatch phenomenon may occur when the preforming bar is prepared, and therefore the attenuation and bend properties are relatively poor.
The attenuation of a silica optical fiber at 600 nm to 1600 nm is mainly due to Rayleigh scattering, and the attenuation caused by the Rayleigh scattering αR can be calculated according to the formula below:
      α    R    =                    1                  λ          4                    ⁢                        ∫          0                      +            ∞                          ⁢                              R            ⁡                          (              r              )                                ⁢                      P            ⁡                          (              r              )                                ⁢                      rdr            /                                          ∫                0                                  +                  ∞                                            ⁢                                                P                  ⁡                                      (                    r                    )                                                  ⁢                rdr                                                          =                  R                  λ          4                    +      B      where λ is the wavelength (μm), and R is the Rayleigh scattering coefficient (dB/km/μm4); P is the light intensity; and when the Rayleigh scattering coefficient is confirmed, B is the corresponding constant. Therefore, as long as the Rayleigh scattering coefficient R is determined, the attenuation caused by the Rayleigh scattering αR (dB/km) will be obtained. On one hand, the Rayleigh scattering is caused by density fluctuation, and on the other hand, the Rayleigh scattering is caused by concentration fluctuation, so that the Rayleigh scattering coefficient R can be expressed as:R=Rd+Rc where Rd and Rc respectively represent the changes in the rayleigh scattering coefficient caused by density and concentration fluctuations. Rc is the concentration fluctuation factor, which is mainly affected by the doping concentration of the glass portion of the optical fiber. Theoretically, the less Ge and F or other dopants are used, the smaller Rc will be, which is also the reason why some foreign enterprises now adopt a pure silica core design to achieve the ultra low attenuation performance.
However, we need to notice that the Rayleigh scattering coefficient also includes another parameter Rd. The Rd is related to the fictive temperature TF of glass and varies with the structural and temperature changes of glass. The fictive temperature TF of glass is a physical parameter representing the structure of glass, and is defined as a temperature corresponding to an equilibrium state reached by the structure of glass without readjustment after the glass is rapidly cooled to room temperature from a certain temperature T′. When T′>Tf (the softening temperature of glass), the structure of glass is easy to adjust because the viscosity of glass is low, thus the glass is in an equilibrium state at every moment, so that TF=T′; when T′<Tg (the transition temperature of glass), the structure of glass is hard to adjust because the viscosity of glass is high, thus the structural adjustment of glass lags behind the temperature change, so that TF>T′; and when Tg<T′<TF (the softening temperature of glass), the time required for the glass to tend to equilibrium is shorter, and is particularly related to the glass components and the cooling speed, so that TF>T′ or TF<T′.
If the pure silica core design is used, in order to ensure the total reflection of the optical fiber, an F-doped inner cladding with relatively low refractive index must be used for matching, so as to ensure that there is an enough difference in the refractive indexes between the core and the inner cladding. For such a pure silica core design, the viscosity of the core is higher while that of the inner cladding doped with plenty of F is lower, thus the structural viscosity matching of the optical fiber is imbalanced, and the virtual temperature of the optical fiber with a pure silica core structure increases rapidly, leading to an increase in Rd. In this way, not only will the advantages brought by reduction of Rc be offset, but also the reverse abnormalities of the optical fiber attenuation may be caused.
To ensure that the viscosity of the core matches with the viscosity of the outer cladding of the optical fiber with pure silicon dioxide core, we can optimize the viscosity of the core by doping alkali metals into the core. In U.S. Publication No. US2010/0195966A1, the method of adding alkali metals into the core is used, wherein under the condition of maintaining the pure silica core of the optical fiber, the problem of the increase in Rd caused by viscosity mismatch is solved by changing the viscosity of the core of the optical fiber and the time for structural relaxation of the core, thus overall reducing the Rayleigh scattering coefficient of the optical fiber. Although the attenuation of the optical fiber can be reduced effectively by using this method, the process and preparation are relatively complicated, core rods need to be treated in multiple batches, and the requirement for the doping concentration of alkali metals is very high, so that this method is not favorable for large-scale preparation of optical fibers.
Chinese Application No. CN201310394404 provides a design of an optical fiber with ultra low attenuation, wherein the design of a pure silicon dioxide outer cladding is used, but because a typical step profile structure is used, a trench cladding design is not used for optimizing the bend properties of the optical fiber, and the core is not doped with Ge, so that a viscosity mismatch phenomenon may occur when the preforming bar is prepared, therefore the attenuation and bend levels are relatively poor.
U.S. Pat. No. 6,917,740 provides a pure silica core optical fiber of which the performance is improved by utilizing the viscosity changes, wherein the core is doped with plenty of F and Cl, and the contributions of the doped F and Cl to the viscosity of the core are utilized to reduce the Rayleigh coefficient of the optical fiber. Moreover, the optical fiber described in the document does not relate to the profile design, and the core is not doped with Ge.
U.S. Publication No. US2010/022533 provides a design of an optical fiber. In order to obtain a lower Rayleigh coefficient, the design of a pure silica core is adopted. In such a design, the core is not co-doped with germanium and fluorine, and the fluorine-doped silicon dioxide is used as the outer cladding. For such a design of a pure silica core, a complicated viscosity matching must be conducted inside the optical fiber, and a very low speed is required in the drawing process, so as to avoid attenuation increase caused by defects inside the optical fiber due to high-speed drawing, so the manufacturing process is extremely complicated.
For a common ultra low attenuation single-mode optical fiber, the outer cladding is completely doped with F. From the perspective of optical fiber optics, such a design is relatively simple, and as long as the refractive index difference between the outer cladding and the core is ensured, the total reflection requirement of the optical fiber can be met. But at present, there are three main factors limiting the manufacturing costs of the optical fibers with ultra low attenuation: first, the alkali metal doping process of the core requires to precisely control the types and concentration of metal ions, so that the manufacturing cost is high; second, the preforming bar purely doped with F is small, so that the drawing process is complex; and third, the F doping process is used for the optical fiber purely doped with F, so that the manufacturing cost is very high. It is estimated preliminarily based on the current market price that the price of an F-doped tube is 5-8 times that of the pure silicon dioxide tube. According to the calculation based on the preliminary relation that the cost for the F-doped material is 6 times that of the pure silicon dioxide material, if the thickness of the F-doped layer is properly reduced through reasonable process design, the manufacturing cost of the optical fiber will be reduced significantly. If the F-doped material is only used at positions where the optical fiber diameter is from 30 to 80 μm, and the common pure silicon dioxide material is used from 80 to 125 μm, then the material cost for such a design will be reduced by 40% when compared with the cost for the traditional optical fiber with ultra low attenuation completely doped with F; and if the F-doped material is used from 30 to 60 μm, and the common pure silicon dioxide material is used from 60 to 125 μm, then the material cost will be reduced by 65%.
Through the above analyses, we can find that the process design of an optical fiber with ultra low attenuation by using a non-pure silica core and a partially fluorine-doped cladding is feasible. However, due to the influence of the first two limiting factors, how to control the optical parameters of the optical fiber under such a design is the final challenge faced by us.
If the pure silicon dioxide that is not doped with fluorine is used as the outer cladding material, there will be three problems.
First, restraint of fundamental mode cutoff: the refractive index difference between the outer cladding material and the core material is too small, so the fundamental mode leakage of the optical fiber will be caused, and the attenuation of the optical fiber will be influenced. Therefore, for the ultra low attenuation fiber designed by using the outer cladding material not doped with F, the fundamental mode leakage must be restrained at the middle position between the outer cladding and the core through reasonable fiber profile design.
Second, consideration of viscosity matching: if no viscosity optimization design is conducted for the outer cladding material, the viscosity will mismatch with the viscosity gradient of the inner cladding and the core, so we control the viscosity of the core by doping with alkali metal ions; and optimize the external glass viscosity of the core through different fluorine doping concentrations of the inner cladding and the trench cladding, so as to reduce problems such as defects at interface positions and virtual temperature rise, and to reduce the attenuation of the optical fiber.
Third, consideration of optical profile matching: if the pure silicon dioxide glass is used as the outer cladding material, the doping concentration of each portion is limited when the viscosity matching design is considered, while in order to ensure that the optical parameters of the optical fiber meet the parameter requirements of G.652 or G.654 optical fiber, i.e. to ensure that the MFD, dispersion and bend properties of the optical fiber meet the standard requirements, we must consider the optical profile design. This requires us to overall consider the optical design of the optical fiber when we design the viscosity, thus the difficulty in process realization is increased.
In conclusion, at present, no relevant patents disclose an optical fiber with ultra low attenuation and large effective area that has all of the following characteristics and can meet the ITU-T G.654.B or D standard requirements: 1, the core is a non-pure silica core, and is doped with germanium; 2, the core is doped with alkali metals to optimize the viscosity of the core; 3, a sunken structure exists between the core and the outer cladding to restrain the fundamental mode cutoff; and 4, the outermost glass material of the optical fiber with ultra low attenuation is silicon dioxide glass not doped with fluorine.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.